I don't see why any of the guys familiar with the playbook couldn't call plays.
Calling plays is more about having a feel for the game than actual knowledge of the system. I'd say Jason Garrett has a huge wealth of knowledge about the offensive side of the ball but he lacks a feel for the game. Felix rips off a 20 yard run and Dallas proceeds to pass its way into a punt.
Find the guy on the defensive side of the ball who happens to be most familiar with the system and let him call plays. I seriously cannot see things getting worse.